The effect of dose fractionation on the induction of translocations by 400 and 600 rad X-rays in spermatogonia of guinea-pigs and hamsters was investigated cytologically. Three types of fractionation were used, dividing the dose into (a) two equal fractions 24 h apart, (b) two equal fractions 8 weeks apart, and (c) eight or twelve equal fractions of 50 rad, at intervals of one week. The two species responded similarly throughout, but gave lower translocation yields than the mouse. The effects of the first and third types of fractionation were similar to those described previously in the mouse, and suggested that a first radiation dose modifies the spermatogonial population so that its sensitivity to a dose 24 h later is altered, and that repeated radiation doses result in development of resistance to translocation induction. After 8-week fractionation the results suggested that in guinea-pigs and hamsters the spermatogonial population had not returned to normal by 8 weeks after the first dose, whereas in the mouse normal sensitivity had returned by this time. The results, reported previously, of single doses of X-rays suggest that the spermatogonial population consists of fractionated doses in the mouse suggest that the sensitive and resistant types represent different phases of the same cell type rather than two distinct types of cell. In the guinea-pig and hamster this question remains open.